Many circuits have been devised for driving a heavily capacitive line to a value approaching the system power supply voltage, V.sub.CC. However, all the circuits heretofore devised possessed various shortcomings described below.
The simplest driver circuit for pulling up a capacitively loaded line to a value approaching V.sub.CC is a simple resistor pull up, i.e., a resistor connected between V.sub.CC and the capacitively loaded line. In order to rapidly pull-up the line, the value of the resistor must be kept small (so as to achieve a small RC time constant). However, if the value of the resistor is kept small, the power dissipated in the resistor when the line is held at a low voltage level (whereupon a large current flows through the resistor), is considerable. Of course, a larger value of the resistor would lessen power dissipation, but at the expense of pull-up speed (higher RC time constant).
A more sophisticated driver circuit for driving a capacitively loaded line is a transistor pull up, wherein a transistor is connected between V.sub.CC and the capacitively loaded line, for example, by connecting the collector of the transistor to V.sub.CC and the emitter to the capacitively loaded line. A biasing resistor is connected between V.sub.CC and the base of the transistor. The transistor, acting as a low impedance source, rapidly pulls up the line. However, since there is always a voltage drop between the base and emitter of a transistor (commonly referred to as V.sub.BE), the transistor driver can only pull up the line to a voltage value of one V.sub.BE drop below V.sub.CC.
The prior art has proposed two modifications of the transistor pull up for pulling up the line the remaining V.sub.BE ; however, neither solution is entirely acceptable. The first solution is to use a driver circuit power supply voltage higher than the system power supply voltage. The capacitively loaded line may then be driven to a value approaching the system power supply voltage despite the transistor base-emitter voltage drop because the transistor is driven by a higher voltage power supply. This solution is not acceptable because of the added complexity and expense of maintaining two separate power supplies in a given system. A second modification is to add a resistor between V.sub.CC and the line, for pulling up the line the remaining V.sub.BE voltage. Such a modification is also unacceptable because the use of a resistor involves the same speed-power trade off involved in the simple resistor pull up discussed above. Briefly, if the resistor is large, pull up time is long, while if the resistor is small, power dissipation is excessive.
Another prior art solution for pulling up a capacitively loaded line to a value approaching V.sub.CC is capacitor pull up. In capacitor pull up, a transistor is used to actively drive the line by connecting the collector thereof to V.sub.CC and the emitter thereof to the line. The base of the transistor is connected to the first end of a capacitor. The base of the transistor is also connected to V.sub.CC through a biasing resistor, so as to permit current to flow into the capacitor. In order to pull up the line, the other end of the capacitor is pulsed, so as to charge up the capacitor by means of an AC current flowing through the biasing resistor and into the capacitor. Then, the pulsed end of the charged capacitor is raised to V.sub.CC. The base of the transistor thus rises above V.sub.CC because of the voltage built up on the charged capacitor, so that the emitter is maintained at or about V.sub.CC, despite the transistor base-emitter voltage drop. The line is thus pulled up to a value nominally approaching V.sub.CC.
Unfortunately, capacitive pull up is not acceptable for use in high density integrated circuit construction, since each driver requires either an external capacitor, or an "on-chip" capacitor. An external capacitor takes up an excessive amount of space, and requires an individual connection to the driver circuit. An "on-chip" capacitor requires an excessive amount of chip area. Since the line driver may be employed in a digital system having a large number (e.g., several hundred) of line drivers, the use of capacitor pull up requires a bank of several hundred external capacitors or several hundred "on-chip" capacitors. Clearly neither solution is acceptable as the space required by such large numbers of capacitors, either on or off chip, is inconsistent with the high density requirements of modern digital systems.
Moreover, capacitive pull up requires additional circuitry for limiting the voltage built up on the capacitor, so that the first end of the capacitor (and the base of the transistor connected thereto), does not drift all the way up to V.sub.CC when the other end of the capacitor is not pulsed. If the base of the transistor drifts up to V.sub.CC then the transistor will turn on and the line will be driven at improper times. Circuitry must be incorporated to limit the charging of the capacitor so that the base of the transistor doesn't rise to V.sub.CC when the other end of the capacitor is not pulsed. The requirement for such limiting circuitry adds to the complexity of the driver and requires an excessive amount of area when hundreds of such limiting circuits must be incorporated on or off chip. As a consequence of the above limitations, capacitive pull up has not been used in integrated circuit technology.